1. Field of the Invention
The present invention relates to an image forming apparatus such as, for example, a printer or a copying machine, and more particularly to an image forming apparatus having an arrangement of a plurality of image forming parts and an image formation control method in the same.
2. Related Background Art
In recent years, a printer as an image output terminal has rapidly become popular. Particularly, with an advance of colorization, there are increasing demands for improving stability of printed image qualities in a color printer and for equalization of color image qualities among color printers. Particularly, color reproducibility on a printed image is required to have advanced stability of image reproduction independent of a change in the installation environment, a change with time, and a difference in printer type. In an electrophotographic image forming apparatus such as a laser beam printer, however, image reproducibility fluctuates due to a change in environmental condition where the apparatus is placed or deterioration with time of a photosensitive member or developer. Therefore, it cannot satisfy the high desired values over the long term if there is no change in the initialization. Accordingly, in this type of color printer, it is common to conduct a feedback control for maintaining the image density optimally.
The feedback control is performed as described below. First, a density patch is formed on a cyclically moving member such as, for example, a photosensitive member, an intermediate transferring member, or a transferring and transporting belt and then a density of the formed density patch is measured. A control factor of the patch density is then controlled in such a way that the density of the density patch is close to a target density, considering a surrounding environment, deterioration with time, and a nonuniformity in solid. There is also suggested a method of forming a density patch on a recording medium such as recording paper and measuring the patch density on the recording medium to perform the same control.
For example, Japanese Patent Application Laid-Open No. H1-169567 discloses a method of measuring a density of a density patch and controlling an exposure condition or a developing bias condition of a laser beam to achieve a desired image density. As a density patch in this case, there is a density patch of an unfixed developed image after a developing process or an image density patch after a fixing process. The reason for using the image density patch here is that it enables an evaluation of the image quality including density fluctuations in the transferring process or the fixing process due to monitoring the image in the same condition as one a user obtains finally. As the feedback control using the density patch, there have already been known a density control for determining a control factor affecting image characteristics such as the highest density, a line width, fogging, or the like (hereinafter, referred to as Dmax control) and a halftone control for correcting linearity (gamma characteristic) of a halftone reproduction (hereinafter, referred to as Dhalf control), as disclosed in, for example, Japanese Patent Application Laid-Open Nos. H7-209934, H10-39555, H11-119481. The Dhalf control for controlling the halftone reproducibility is generally performed after the Dmax control in order to use a result of the Dmax control. Thus, a gamma correction after controlling the highest density to a predetermined value keeps the linearity and regularity of the density.
A normal printing operation cannot be performed during the above calibration, thus the user have to wait for the finishing the calibration to execute the printing operation. Therefore, it is required to reduce the time for the calibration as greatly as possible. In addition, in calibration for printing a test pattern on paper or other recording medium, it is required to minimize a quantity consumed of recording medium (recording paper), which is a user resource.